Rates of morphological evolution in Captorhinidae: an adaptive radiation of Permian herbivores.

Neil Brocklehurst

doi:10.7717/peerj.3200

Abstract

The evolution of herbivory in early tetrapods was crucial in the establishment of terrestrial ecosystems, although it is so far unclear what effect this innovation had on the macro-evolutionary patterns observed within this clade. The clades that entered this under-filled region of ecospace might be expected to have experienced an “adaptive radiation”: an increase in rates of morphological evolution and speciation driven by the evolution of a key innovation. However such inferences are often circumstantial, being based on the coincidence of a rate shift with the origin of an evolutionary novelty. The conclusion of an adaptive radiation may be made more robust by examining the pattern of the evolutionary shift; if the evolutionary innovation coincides not only with a shift in rates of morphological evolution, but specifically in the morphological characteristics relevant to the ecological shift of interest, then one may more plausibly infer a causal relationship between the two.Here I examine the impact of diet evolution on rates of morphological change in one of the earliest tetrapod clades to evolve high-fibre herbivory: Captorhinidae. Using a method of calculating heterogeneity in rates of discrete character change across a phylogeny, it is shown that a significant increase in rates of evolution coincides with the transition to herbivory in captorhinids. The herbivorous captorhinids also exhibit greater morphological disparity than their faunivorous relatives, indicating more rapid exploration of new regions of morphospace. As well as an increase in rates of evolution, there is a shift in the regions of the skeleton undergoing the most change; the character changes in the herbivorous lineages are concentrated in the mandible and dentition. The fact that the increase in rates of evolution coincides with increased change in characters relating to food acquisition provides stronger evidence for a causal relationship between the herbivorous diet and the radiation event.

Highlights

The evolution of high fibre herbivory represents a major step in the establishment of terrestrial ecosystems
Arthropod herbivores were present in terrestrial ecosystems prior to the evolution of herbivory in tetrapods, terrestrial vertebrate herbivores were entering a somewhat under-filled region of ecospace
The link between a supposed “key innovation” and an adaptive radiation must always, to a certain extent, be circumstantial; one may identify the branch in a phylogeny along which the evolutionary novelty likely appeared, and one may identify the location of shifts in rates of evolution and diversification, but conclusively proving a causal relationship between the two is extremely difficult

Summary

Introduction

The evolution of high fibre herbivory represents a major step in the establishment of terrestrial ecosystems. Arthropod herbivores were present in terrestrial ecosystems prior to the evolution of herbivory in tetrapods, terrestrial vertebrate herbivores were entering a somewhat under-filled region of ecospace These early herbivores provide an ideal opportunity to examine the changes in rate and mode of evolution and diversification resulting from evolutionary innovations. Simpson’s adaptive radiation model (Simpson 1953) posits that a “key” evolutionary novelty gives a lineage a selective advantage or allows it to enter a new ecological niche and leads to an increase in morphological diversification or speciation rates. Such a model is often invoked when analyses of diversification rate heterogeneity identify shifts which coincide with an innovation of interest

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